1. Field of Invention
This invention relates to switching power supplies; and more particularly, to such power supplies having improved control characteristics.
2. Description of Prior Art
The switching power supply, of the type to which the invention relates, serves to supply a constant DC voltage with respect to a variety of loads using an input DC voltage which fluctuates. In particular, a voltage resonant type switching power supply converts variations in voltages of semiconductor switches, such as MOSFETS, into sine waveforms which smoothly vary while utilizing an LC resonance. This type of supply is characterized by a smaller switching loss and less noise during switching operation than other types of prior art switching systems.
FIG. 1 is a circuit diagram depicting one example of a prior art switching power supply, wherein an input voltage Vi is applied to the system which comprises dividing capacitors 1,2, constituting a half-bridge circuit, for halving the input voltage Vi; semiconductor switches 3,4, such as MOSFETS, connected in series to both ends of the terminals to which are applied input voltage Vi; excitation power sources 5,6, connected between the gate/source terminals of the MOSFETS 3,4, for driving MOSFETS 3,4; parasitic diodes 7,8 to MOSFETS 3,4; voltage resonant capacitors 9,10 connected between the drain/source terminals of MOSFETS 3,4; a voltage resonant inductor 11, one end of which is connected to the connecting point between dividing capacitors 1,2; a transformer 12 having a primary winding, both ends of which are connected to the connecting point between MOSFETS 3,4, as well as to the other end of voltage resonant inductor 11; rectifying diodes 13,14 having anodes thereof connected to both ends of the secondary winding of transformer 12; snubber capacitors 15,16 connected in parallel to rectifying diodes 13,14; and a choke coil 17 , one end of which is connected to the cathodes of rectifying diodes 13,14.
A filter capacitor 18 and a load resistor 19 are connected between the other end of choke coil 17 and the midpoint of the secondary winding of transformer 12. Choke coil 17 and filter capacitor 18 are combined to constitute an output filter. An output voltage Vo is impressed on both ends of load resistor 19. An output voltage Vh is produced by a half-bridge circuit comprising components of dividing capacitors 1,2 to voltage resonant capacitors 9,10. An output feedback circuit 20 comprises dividing resistors 21,22, reference voltage 23, comparator 24, and an ON time control circuit 25, and is used for changing the ON time of switching of MOSFETS 3,4, corresponding to the difference between the output voltage Vo and the reference voltage 23.
FIG. 2, comprising lines (1)-(8), is an operating waveform diagram used to explain the operation of the system of FIG. 1, wherein line(l) shows an output of excitation power source 5, i.e. a gate drive signal to a MOSFET 3 which is turned ON at a high level thereof; line (2) shows an output of excitation power source 6, vis., a gate drive signal to MOSFET 4 which is turned ON at a high level thereof; line (3) shows a drain source voltage of MOSFET 3; line (4) shows a drain current of MOSFET 3; line (5) shows a drain current of MOSFET 4; line (6) shows a current flowing in the primary winding of transformer 12 and voltage resonant inductor 11; line (7) shows a current flowing in dividing capacitors 1,2; and line (8) shows a current flowing in voltage resonant capacitors 9,10.
As shown in lines (1) and (2), MOSFETS 3,4 are turned ON alternately for a given period of time by excitation power sources 5,6. When MOSFET 3 is turned ON at a time t0, the current depicted in line (4) flows in MOSFET 3. A negative current flows in parasitic diode 7 and works to regenerate the energy to an input. When turning OFF MOSFET 3 at time t1, a resonant state is developed during a period ranging from t1 to t2 by use of voltage resonant capacitors 9,10 in combination with voltage resonant inductor 11.
A voltage at both ends of voltage resonant capacitor 10 is, although the period from t2 to t3 originally assumes the resonant gate, held at substantially zero for parasitic diode 8. To be specific, input voltage Vi is applied to both end of MOSFET 3, while both ends of MOSFET 4 assume substantially zero voltage. At this time, MOSFET 4 is turned ON (time t3) and a switching loss is thereby remarkably reduced (to zero voltage switching). MOSFET 4 is kept ON during the period from t3 to t4. When turning OFF MOSFET 4 (time t4), similar to the period t1-t2, the resonant state is developed once again, with the result that the voltage at both ends of MOSFET 4 rises while being resonated (period t4-t5).
During the period t5-t6, the zero voltage is held at both ends of voltage resonant capacitor 9 for the sake of parasitic diode as seen during the period t2-t3. Namely, input voltage Vi is applied to both ends of MOSFET 4, and both ends of MOSFET 3, assume substantially zero voltage. At this time, MOSFET 3 is turned ON (time t6), thereby considerably decreasing the switching loss (zero voltage switching). These operations are repeated.
The current depicted in line (6) flows in the primary winding of transformer 12 and in voltage resonant inductor 11, to transfer the energy to the secondary winding. Hence, when turning ON MOSFETS 3 and 4, the voltage applied thereto is almost zero. On the other hand, in an OFF state , the waveform of the voltage across the MOSFETS 3,4 is moderate due to the resonance.
For these two reasons, the switching loss is remarkably decreased and noise caused during switching operation is reduced.
Control over the output voltage is effected by means of the output feedback circuit 20 to change the ON time of switching of MOSFETS 3,4, corresponding to the difference between the output voltage and the reference voltage. More specifically, the ON time of MOSFETS 3,4 is changed by use of ON time control circuit 25 in accordance with the output error detected by dividing resistors 21,22 in combination with reference voltage 23 and comparator 24. That is to say, if the output voltage is small, the ON time is increased (both the switching cycle and the duty become large) and considerable input power is conveyed to the output side. On the other hand, if the output voltage is large, the ON time is decreased. Note that the OFF time is determined by the resonant frequency and is therefore invariable. As a result, the switching fequency has to be changed for output control.
According to the prior art switching power supply discussed above, if the output filter,consisting of choke coil 17 and filter capacitor 18, is sufficiently large, the secondary side can be replaced with a current source Io. If the half bridge circuit, consisting of the components ranging from dividing capacitors 1,2 to voltage resonant capacitors 9,10, is replaced with a DC-AC converter, the circuitry thereof can be expressed by the simplied circuit,shown in FIG. 3, comprising input voltage Vi, DC-AC converter 26, voltage resonant inductor 11, load resistor 19 and current source Io. However, the FIG. 3 arrangement is disadvantageous in that load resistor 19 and voltage resonant inductor 11 are connected in series which causes an increment in output impedance so that the output voltage Vo is adversely influenced by fluctuations in the load.